docs: add operating manual, fix cross-references after doc restructure

- Commit CLAUDE.md + AGENTS.md so a fresh clone has the operating manual and
  the ABI/no-game-logic rules (trimmed of stale phase status; the personal
  pd_port path removed; points at the split docs).
- Sweep every stale doc reference across the tree after the restructure:
  PCPortResearch.md -> internals.md / dev/findings.md,
  PORT-LEARNINGS.md -> porting-notes.md,
  AGENT-WORKFLOW.md -> dev-process.md,
  docs/{GRAPHICS-BACKLOG,HANDOFF-ARCHIVE,GE-ENV-PROBES,LEVEL-*,TEXTURE-GLITCH-ANALYSIS,AUDIT-M6-*,AUDIO-PLAN}.md -> docs/dev/.
- Drop the superseded docs/PLAN-linear-level-sweep.md.
- docs/HANDOFF.md + docs/BACKLOG.md stay local-only (rolling working files);
  CLAUDE.md now handles their absence in a fresh clone.

docs/dev/LEVEL-PLAYTEST.md

@@ -1,6 +1,6 @@
 # LEVEL-PLAYTEST — WS6 human completion-validation checklist
 
-Run **after** `docs/LEVEL-STATUS.md` is all-21 load+render PASS. This is
+Run **after** `docs/dev/LEVEL-STATUS.md` is all-21 load+render PASS. This is
 the part that needs real input — a human plays each level start to finish
 and confirms it matches retail. Findings feed a WS5-style triage round
 (log each as a crash-class row or a new Dxx candidate).